drm/amd/display: Reset MPCC muxes during init

During HW initialization, instead of assuming or
detecting the existing MPCC mux configuration and
then removing existing planes, reset all the MPCC
muxes.

Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>